Method of etching aluminum and aluminum base alloys



Patented Sept. 1, 1953 METHQD. 015' E'LCHING. ALUMINUM A UM NUM BASE ALLOYS Vladimir Dvorkovitz, Chicago, and John R.. Swihart Ghicago. Heights, 111., assignors to The niverse} Qorporation, a corporation of Illinois Nit-Drawing. Application December 9, 1950,

Serial No. 200,117

11 Claims.

This invention relatestto a. method: of etchin aluminum and aluminum base; aiu ysl.

In many instances. whereartjicles. of: aluminum or aluminum base alloys; are tahrijcated it is, necessary to etch the; surfiace-of. the; article.- either as n id in fabricationofthe article. to provide a e for further finishin -co e give avfinal decorative finish. The. usuat industrial; practice in etching e materials: is to treat-the; surface of the article, with, a, hot; aqueous: soluti n coirta ninsan etch n alkali. such. as sodium hyx d p ssium. hydroxide. trisod um choc: p t s da ash. r s milar-mat r als;- Qrm xtures of these chemicals in varying proportions. Because of the chemical activity of the aluminum, it reacts with these alkaline etching chemicals to form aluminum reaction products as may be represented; by the reaction with sodium hydroxide:

Although such practicesin'the past have been. effective in etching the metals, these methods are not entirely satisfiactor y and the; n'iethod has certain inherent disadyanl figfia Probably the most importantv of thesedisadvanta es is the formation of a, hardrock-like scale. or m ss in the etching tank whichisdeposited on the sides of the tank, on the heating units in the tank and on other surfaces. This scale is ordinarily so hard and tenaciousthat lt is quite, diifiou t to remove. Generally this scale canonly be, re,- moved by chipping it; off with. hammers. or; other tools. This not onlycauses loss, oi. time .andexpensive labor costs but thetb leaking looseof the scale often damages the equipment. The scale is apparently formed. whenthe concentration of, the alkali in theretching solution becomes reduced due to the; reaction oi the alkali with the, metal. When this happensathe: sodiumalumia re ionproduct. reacts with water t P cip tate huninummlirdroxide. accordin o h equation:

The aluminum, hydroxide apparently thenreacts further to precipitate alumina according to the following eq ation:

The alumina, isapparently the principal constituent of the hard-rock-like scale;

It is known that; the scale formation can be postponed for a short period by the addition of fresh alkali such as sodium hydroxide to the solution as the alkali, in the solution is used up. However, as, the concentration of aluminum in the solution. increases, the addition of fresh alkali must be similarly increased in amount until these additions; become so large that they upset the control necessary for the production of av uniformly etched product. Thus the attempt to solve the problem by additions of fresh alkali s not. o m y feasible- As can be noted from the aboveequations, the reaction between the water and the sodium aluminate: regenerates; some sodium, hydroxide. While this is of some advantage; it is more than counteracted by the objectionable scale that is also formed; Furthermore this regeneration of alkali cannot be controlled so that as the work is withdrawn from; the: etching bath the regeneration often occurs in the film adhering to the work so that the increased concentration of alkali produces a non-uniform surface. Fur

thermorethe scale that is formed often adheres.

oz/gal. ozlgal. oz/gal'. of'avail. NaO H Precipitate of NaOH regenerated A1203.3H2O' Freshsolution 10. 6 0. 0 Used solution 1.2 0.0 Used solution, after 92 hour 3.0 1.8 7.1 Used solution, after 8 hours. 6. 1 4. 9 19. 2

1 Slight.

The above-table of examplesis only indicative of the results achieved since the degree of reversionis controlled by temperature, additions of alkali as upkeep, the time of the etching process and other variables.

Applicants have successfully solved the problem of etching aluminum and aluminum base 4 as arabinose, rhamnose, galactose and fructose, which is a ketone sugar, starch and gum arabic are useful in controlling precipitation but sometimes excessive foaming occurs with these latter ll y W hout h f rm i n of r ck-like le 5 two materials. The most desirable inhibitors are by employing a method wherein the article of gluconic acid, mucic acid, the alkali metal salts aluminum or a u num a y is trea w t a of these acids, glucoheptonic lactone, gluco lachot aqueous solution comprising an etching altone, corn starch, sorbitol and gum arabic. Glukali and a small amount of a water soluble satuconic acid or its salts appears to be the most deted aliphatic p y y y inhibitor Compound sirable inhibitor because of its stability in heated described in ea er de l here ters solutions, uniformity of results obtained and new method Operates either by p event ng the commercial availability. If desired, the gluconic reversion of the sodium aluminate to sod u yacid may be added as such to the etching solution droxide and alum a y controlling this where it is immediately converted to the alkali version so that any scale formed iS in ge metal salt or the alkali metal salt itself may be granular and u non-adherent t0 the su added. This is, of course, true of any of the other contac d by e b In this letter instance acids. Glycerine and tartaric acid inhibit scale the alumin p cipi f rms as a lu wh formation but are not preferred as their effectivecan be easily washed away due to its non-adness i of h t d ti herent character. The mechanism by which these organic ma- The advantages of the new method of etching terials function to control the precipitation of aluminum as described herein are nu O alumina is not completely understood. However, Thus the invent o prevents costly damage the improved results of the new method have caused by the former necessity of cleaning the b proven i practicalapplicafions q p t y ppin e scal from this p- Several experiments were made using sodium ment. Furt e O the operetOl 0f the etching gluconate in various amounts and as compared bath can exercise better and closer control over with ntr 1 employing di hydroxide and the process so th t he number of rejects are trisodium phosphate and a second control emp y edu ed- The plant shutdowns necesploying sodium hydroxide alone. In each test sa y for r pa a to r p a e the sp nt t in aluminum was etched at 180 F. in a solution consolutions are kept to an absolute minimum. Also, taining 10 ounces per gallon of the etching chemithe full benefit of the chemicals used in the new cals. After no more aluminum would react with process is obtained as objectionable side reactions the solutions, these solutions were allowed to which tend to reduce the efficiency of the process stand for eighteen hours after which they were are kept to a minimum. Other advantages are examined for the effectiveness of the gluconate in the improvement and uniformity in the quality preventing scale precipitation and adherence. of the work produced and the ease of disposal of The following results were obtained. the solid granular waste when the precipitate occurs. Furthermore, subsequent chemical treatments of the etched surfaces are often much Appeab g g easier than was true when prior etching processes solution 10 oz/gal. pf Mix Perance of Precipiwere used. All of these improvements and adoon'mmmk cent 552 il n vantages increase the economy, efiiciency and tainer ease of operation of the etching process.

The polyhydroxy inhibitors coming within the 4.5 1 96 above described class include polyhydric alcohols 2 d 3 Yes having the formula CH20H(CHOH)|1CH2OH, 95 wherein a is at least 1, starches having the gen- 3 n3 5 eral formula [Cs(H20)5], sugars having the 4 fi 98 general formula CHO(CHOH)tCI-I2OH wherein Z) gagfifj Pfffflj 2 t is at least 3, polyhydroxy monobasic acids having 5 the general formula coomcnoiiiccmon wherein c is at least 4, polyhydroxy dibasic acids having the general formula In order to control scale formation and utilize COOH(CHOH) dCOOH regenerated sodium hydroxide, two solutions were wherein d is at least 2, and acids having the charged at 8 ounces per gallon in separate steel generaly formula CI-IOICHOHMCOOH wherein e containers heated with steam coils at 160 F. is at least 4. Among the preferred inhibitors are Aluminum was dissolved for a period of 240 hours. sorbitol as the polyhydric alcohol, corn starch, 0 This was equivalent to four times the theoretical glucose and lactose as the sugars, gluconic acid amount which would react with the caustic presas the monobasic acid and mucic acid as the dient. At the end of that time, each container was basic acid. The salts or lactones of these subexamined for scale and precipitate. Th followstances may also be used. Similarly, gum arabic ing results were noted,

strliclg Boa-fiilgifloilix Percent AmounlaggPrecipi- Condltloiizi1 t08f Preclpi- 1 IgTTZggIIIIIII: 13g Tank%full. Hard rock-like mass. 2 {Purifiedsodiumglu- 2 }Tank}full Sludge.

conate.

which contains glucuronic acid from the family having the last previously described structural The steam coils in solution 1 were coated with a /2 inch of a hard white crystalline deposit which formula may also be used. Other sugars, such was removed only with difiiculty. No solid residue. was observedson the con:- used to heat solution 2.

A similar. experiment usinga mixture containing 97% sodium. hydroxide and; 3% technicalsodium gluconate gave results similar to those obtained. above with solution2.

A- further advantage obtained in using. a suitable hydroxyv compound as described herein is the. elimination or the substantial reduction of the problems involved. in removing smut often formed during aluminumetching. This smutis ordinarily silicon, carbon, copper on. other materials left at. the surface of the article being etched after. removalv of the aluminum by the etching solution. The. removal of this smut often requires additional treatment in an acid bath. When aluminum and. aluminum base" alloys are treated accordingto the method of this invention the smut that remains can be removed much easier so that the removal time is reduced byv /3 to as compared with similar articles etched in a bath according to the old methods. In many instances, the new method leaves the smut in such a condition that it can be easily rinsed off so that the usual acid treatment is unnecessary.

In testing the methods of this invention it has been observed that crude and technical grades of gluconates and gluconic acid give somewhat better results than the purified material although the reason for this is not understood. The following data illustrates the differences in the crude or technical grade gluconate and the purified material.

In. contrastv to this the materials; used in the present methods produced the. following; comparative results:

-' gpmpafiztivle 11316 0 CB 9 Material Foi-matibnl Hours NaOH only, control 0 Glucoheptonic lactone. +16 Tartaric acid +2 Mucic acid +8 U. S. P. gluconic acid +16 Corn starch +11 Glucolactone +16 Glucose +4 Lactose +4 Glycerine. +1 Sorbitol +16 Gum arabio. +16

A preferred composition for use in applicants? method is made up of 96% by' weight of sodium hydroxide and 1% of technical 50% gluconic acid. This acid, of course, is converted immedi-- ately to sodium gluconate.

In the preferred method, the solution is main-- tained at a temperature of from 120 F. to 210 F during the treatment and the polyhydroxy compound is present in an amount of 0.5% to 10% by weight of the combined alkali and the com-- pound. In the preferred solutions the alkali is: present in an amount of 4.0% to 10.0% by weight of the solution.

Having described our invention as related to the embodiments disclosed herein it is our inten- Solu- 10 oz./ga1.0f mix com- Per- Precipitate Obtion posed of cent Aluminum added served After- 1 NaOH 100 Theoretical amount lhour.

for caustic present. NaOH 95 2 Purified sodium glu- 5 do hours.

conate. NaOH 99.5 3 Purified sodium glu- .5 do l8ho11rs.

conate. NaOH 99.5 4 Crude sodium glu- .5 do 36hours.

conate. NaOH 97 5 Crude sodium glu- 3 do None formed for conate. 170 hours. aOH 97 6 Tech. sodium glu- 3 -.do 96hours.

conate.

Comparative Time of Scale Formation, Hours Material NaOH only, Control Itaconic acid Sorbic acicl Aconitic acid 2,4 dimethylene glucomc acid. Lactic acid Citric acid Tetra sodium salt of ethylene d1amme tetra acetic acid Ethylene glycol tion that the invention be not limited by any of the details of description unless otherwise specified but rather be construed broadly within its spirit and scope as set out in accompanying claims.

We claim:

1. In the etching of articles of aluminum and of aluminum base alloys, the method which comprises treating the surface of said article with a hot aqueous solution comprising an etching alkali and a small amount of water-soluble saturated aliphatic polyhydroxy compound in a proportion such that the formation of a hard adherent precipitate is avoided for an extended period of time, said compound being selected from the class consisting of polyhydric alcohols of the formula CI-IzOI-HCHOHMCHzOH wherein a is at least 2, starches of the formula (C6H10O5)z, polyhydroxy monobasic acids of the formula COOH(CHOH) cCH2OH wherein c is at least 4, polyhydroxy dibasic acids of the formula COOH CHOH COOH wherein (Z is at least 3, gum arabic and the alkali metal salts of said acids.

2. The method of claim 1 wherein said polyhydroxy compound is present in an amount of 0.1% to 20.0% by weight of the combined alkali and said compound.

3. The method of claim 1 wherein said solution is maintained at a temperature of 60 to 212 F. during said treatment.

4. The method of claim 1 wherein said polyhydroxy compound is present in an amount of 0.1% to 20.0% by weight of the combined alkali and said compound, the solution is maintained at a temperature of 60 to 212 F. during said treatment, and the alkali is present in an amount of 2.0% to 20.0% by weight of the solution.

5. The method of claim 4 wherein said alkali comprises sodium hydroxide and said polyhydroxy compound comprises sodium gluconate.

6. In the etching of articles of aluminum and of aluminum base alloys, the method which comprises treating the surface of said article with an aqueous solution containing ounces per gallon of a mixture of 98% by weight of sodium hydroxide and 2% by weight of sodium gluconate while maintaining said solution at a temperature of about to F.

7. The method of claim 1, wherein said aliphatic polyhydroxy compound is glucoheptonic lactone.

8. The method of claim 1, wherein said aliphatic polyhydroxy compound is gluconolactone.

9. The method of claim 1, wherein said aliphatic polyhydroxy compound is gluconic acid.

10. The method of claim 1, wherein said aliphatic polyhydroxy compound is music acid.

11. The method of claim 1, wherein said aliphatic polyhydroxy compound is sorbitol.

VLADIMIR DVORKOVITZ. JOHN R. SWIHART.

References Cited in the file 0! this patent UNITED STATES PATENTS Number 

1. IN THE ETCHING OF ARTICLES OF ALUMINUM AND OF ALUMINUM BASE ALLOYS, THE METHOD WHICH COMPRISES TREATING THE SURFACE OF SAID ARTICLE WITH A HOT AQUEOUS SOLUTION COMPRISING AN ETCHING ALKALI AND A SMALL AMOUNT OF WATER-SOLUBLE SATURATED ALIPHATIC POLYHYDROXY COMPOUND IN A PROPORTION SUCH THAT THE FORMATION OF A HARD ADHERENT PRECIPITATE IS AVOIDED FOR AN EXTENDED PERIOD OF TIME, SAID COMPOUND BEING SELECTED FROM THE CLASS CONSISTING OF POLYHYDRIC ALCHOLS OF THE FORMULA CH2OH(CHOH)CH OH WHERIN A IS AT LEAST 2, STARCHES OF THE FORMULA (C6H10O5)X, POLYHYDROXY MONOBASIC ACIDS OF THE FORMULA 